I have two and keep my carbon and GFO separated since they have both different flow requirements and different useful lives. I use one MJ600 powerhead through a tee so each is fed separately andcan be valves off and changed without affecting the other. When you mix the media one or the other gets tossed before it is exhausted or you stretch the other one past its prime. Carbon is pretty cheap, GFO is not.
Spectrapre sells a dual reactor with separate feeds and valves if you will be using larger amounts of media that works real well. For smaller amounts I prefer the Phosban reactors though as they are more compact.